Finished workpieces are frequently quenched after being heated to impart to the workpiece a specific range of hardness, type of microstructure and other metallurgical and physical properties that are desirable for its intended use. Workpieces constructed of low carbon steel are typically quenched after being carburized during the finishing of the part. During carburization, the workpiece is heated to a high temperature and introduced into a carbon-rich atmosphere for diffusing a specific amount of carbon into the workpiece to control its surface (case) hardenability. After being carburized, the hot workpiece is quenched to rapidly lower its temperature to control the final microstructure formed during quenching to harden the workpiece, increase its fatigue resistance and impact toughness and impart other desirable physical properties to the workpiece.
Alternatively, work pieces made of high carbon steel (typically 1.0% C) need only be heated to the proper temperature for hardening by quenching. Carburizing is not required.
In the quenching of workpieces, a batch of workpieces is typically freely immersed in a tank filled with quenching fluid to reduce the temperature of each workpiece from near the carburizing or hardening temperature to a predetermined final temperature. While immersed, the temperature of each workpiece rapidly drops causing the microstructure of each workpiece to change. More importantly, the rate of cooling is rapid and carefully controlled to precipitate and permanently fix a desired microstructure in each workpiece when it reaches the final temperature. To increase the cooling rate for producing a more beneficial microstructure, such as martensite in steel, each workpiece or the fluid in the tank is typically agitated to induce turbulence in the fluid around each workpiece to increase the rate of heat transfer from each workpiece to the quenching medium.
Unfortunately, during quenching the workpieces may significantly distort or even crack rendering them unusable or typically requiring them to be later machined to their proper finished dimensions. Thin-walled workpieces, such as annular bearing races, are particularly vulnerable to distortion and cracking because their thin cross section causes difficulty in controlling the quench rate necessary to produce the desired final microstructure. As such, the scrapping of cracked or severely distorted quenched parts significantly increases manufacturing costs which in turn reduces profit. Equally financially burdensome is the significant labor and material cost associated with machining usually by grinding the hardened and distorted workpieces to their proper finished dimensions for use.